The present invention relates to a composition for forming a chemically resistant, synthetic, inorganic, refractory fiber.
To date, most of the work in development of a synthetic inorganic fiber suitable for use in reinforcing cementitious matrices has been performed in the glass area. As used here, the term "glass " refers not only to the vitreous, or non-crystalline, structure of the resultant fiber but, also, to a composition containing one or more fluxing agents, usually alkaline metal oxides. The function of these fluxing agents is to reduce the melting temperature of the batch mix making the composition more fiberizable by such methods as continuous pulling and rotary fiberization (extrusion through holes of a spinner disc). At the same time, however, these fluxing agents generally detract from the chemical inertness of the fiber and reduce the service temperature thereof. In applications requiring both chemical and temperature resistance, (e.g., heat treated cementitious product reinforcement) glass fibers containing alkaline metal oxide fluxes can lose 35% or more of their tensile strength with a corresponding loss of reinforcement effectiveness in the cement matrix. This translates into a lowering of the modulus of rupture of the resulting fiber/cement product (i.e., a direct loss of strength of the reinforced product).
Accordingly, it is an object of the present invention to produce a synthetic inorganic fiber which will be both chemically resistant, so as to enable fibrous reinforcement of acidic and alkaline compositions, and will be temperature resistant in that environment, to enable heat treatment of the resultant product. It should be noted that the high temperatures (300.degree. to 1100.degree. F.) present in curing procedures, accelerate not only the chemical combination of silica and lime and the volatilization of organic fiber and excess water, as intended, but also accelerate other chemical reactions such as the alkaline or acidic attack of reinforcing fibers by the matrix. Therefore, a fiber which may be chemically resistant in an alkaline environment and thermally resistant at 500.degree. F. (260.degree. C.) outside that environment, will not necessarily be resistant to the combination of conditions. Also a fiber which is highly alkali resistant may not necessarily be resistant to acid attack. An example of such a fiber is asbestos.
Zirconia (ZrO.sub.2) has been extensively used as an additive in glass formulations as a means of imparting alkali resistance. See for example U.S. Pat. Nos. 3,859,106; 3,966,481; 4,036,654; 4,330,628. In addition, zirconia by itself or as a predominant componet has been used to formulate refractory compositions with relatively high service temperatures. See for example U.S. Pat. Nos. 2,873,197; 2,919,944; 3,035,929; 3,754,950; 3,793,041; 4,053,321; and 4,119,472. As previously noted, however, the suitability one ingredient for independent chemical, and temperature resistance does not insure that the composition employing that ingredient will have the required combined chemical and thermal resistance. The combination of simultaneous chemical and thermal attack can produce a highly corrosive environment. In fact, it was a zirconia containing ostensibly, alkali-resistant glass that proved unsatisfactory for this usage, leading to the present invention. Further, research performed in conjunction with the making of this invention bears this out by showing that not all formulations of the components used in making the fibers of the present invention produce the desired chemical and thermal resistances.
The objects of the present invention are satisfied by a compositional formulation suitable for producing refractory fibers which is virtually free of alkali metal oxide fluxes, comprising from 56 to 76% silica, from 12 to 33% alumina and from 3 to 22% zirconia. Fibers having this basic chemical composition have been found to be essentially chemically inert in both acidic and alkaline environments. Examples of such environment would be acidic solutions in batteries or calcium silicate products, even when those products are heat treated at temperatures of 300.degree. to 1100.degree. F. (150.degree. to 593.degree. C.). These refractory fibers are formulated by impinging a molten stream upon the surfaces of two rapidly rotating spinners. This process for manufacturing spun fibers is actually more efficient when the melt stream is in the range of 3000.degree. F. (1705.degree. C.), making the use of the fluxing agents noted above, undesirable.
These and other features, advantages and characteristics of the present invention will become better understood after a reading of the detailed description which follows.